Figure 1. Survival estimates, for adult and juvenile (first year bird) snail kite in Florida between 1992 and 2001, the error bars correspond to the sampling error. [larger image]

The snail kite (Rostrhamus sociabilis plumbeus) is an endangered raptor that inhabits flooded freshwater areas and shallow lakes in peninsular Florida and Cuba (Sykes 1984, Sykes et al., 1995). The historical range of the snail kite covered over 4,000 km² (2,480 mi²) in Florida, including the panhandle region (Sykes et al., 1995), but is now restricted mainly to the watersheds of the Everglades, Lake Okeechobee, Loxahatchee Slough, the Kissimmee River, and the Upper St. Johns River. These habitats exhibit considerable variation in their physiographic and vegetative characteristics, and include graminoid marshes (wet prairies, sloughs), cypress swamps, lake littoral shorelines, and even some highly disturbed areas such as agricultural ditches or retention ponds (Bennetts and Kitchens, 1997). Three features that remain constant in the variety of selected habitats are the presence of apple snails, areas of sparsely distributed emergent vegetation (Sykes, 1983; 1987), and suitable nesting substrates, all of which are critical to the nesting and foraging success of the snail kite.

Snail kites are dietary specialists, feeding almost exclusively on one species of aquatic apple snail, Pomacea paludosa (Sykes, 1987; Sykes et al., 1995). The snail kite's survival depends on those hydrologic conditions that support these specific vegetative communities and subsequent apple snail availability in at least a subset of wetlands across the region each year (Bennetts et al., 2002). Wetland habitats throughout central and southern Florida are constantly fluctuating in response to climatic or managerial influences, resulting in a mosaic of hydrologic regimes.

The aim of the snail kite project is to monitor the response of the birds to those changes. This research essentially focuses on the most critical demographic parameters: survival, reproduction, recruitment and population growth rate (Bennetts et al., 1999; Dreitz et al., 2001; Bennetts et al., 2002; Dreitz et al., 2002). Because those demographic parameters are so heavily influenced by the behavior of the birds (i.e. their ability to move and select suitable habitats), movement studies constitute the other major aspect of the research. The objectives are twofold: First to evaluate the likelihood of biological hypotheses, which help understand the underlying mechanisms and processes driving the population dynamics of the kites; and second to provide reliable estimates of demographic parameters and movement probabilities, which are helpful for decision making using management models (see below). The statistical framework selected for parameter estimation is maximum likelihood estimation for its well recognized good statistical properties (Burnham and Anderson 1998). The empirical data consists of mark-resight and radio telemetry data.

The long-term data set already available offers the potential to investigate the effect of hydrological variations across time and space. Low water years have substantial effects on the number of nests detected, but do not seem to greatly affect nesting success (Dreitz et al. 2001), which suggest that when the hydrological conditions are unfavorable for kites, birds simply do not breed. Thus far there is no evidence for an effect of local drying event on adult survival, although this aspect needs to be investigated for juvenile. There is no strong evidence for a very substantial effect of a fairly widespread drought event (however, low in intensity) on adult survival (the 2000-2001 drought, fig. 1). In contrast the 2000-2001 drought considerably affected juvenile survival (fig. 1).

It was also interesting to note that it is during the wettest year that the highest estimates of juvenile survival were observed. Bennetts et al. (2002), explain this observation by suggesting that during high water year, more sites are likely to be suitable for foraging. Hence, during the dispersing period, when juveniles are most vulnerable, an increase in the surface area of suitable habitat, would reduce the chances of the dispersing juveniles encountering unsuitable habitat. An important point to make here is that the results provided in this abstract are preliminary. Those estimates will be refined by incorporating the 2003 field data. Nonetheless, the estimates that this analysis provides are already valuable for further modeling effort, in particular in the context of the Across Trophic Level Simulation System (ATLSS), which evaluates the effect of various hydrological regimes on the whole Everglades ecosystem (DeAngelis et al. 1998; DeAngelis et al. 2002). Indeed the present version of the spatially explicit individual-based kite model that will be incorporated into ATLSS (Mooij, Bennetts et al., 2002), is presently lacking robust estimates for survival during drought events.

Another aspect presently under investigation is the response of the bird in terms of movement. By combining radio telemetry and mark-resight data under a multistate modeling framework (Williams, Nichols et al., 2001), we intend to estimate yearly movement probabilities between critical habitats, and test biological hypotheses about the processes driving the long term movement patterns. This information together with some further investigation of within year movement patterns, should help improve the predictive performance (in particular the spatial component) of the spatially explicit individual based kite model.

Contact: Julien Martin, Florida Cooperative Fish and Wildlife Research Unit, University of Florida, Gainesville, FL 32611-0450, email: martinj@wec.ufl.edu

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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